1. Field of the Invention
The present invention relates to varnish compositions for use in offset printing inks.
2. Brief Description of the Prior Art
As general processes of printing, there have currently been extensively used relief printing, lithography (for example, offset printing or the like) and photogravure. Of these printing processes, the offset printing process comprises transferring inked images from a printing plate roll to a rubber blanket and printing the images on paper. The printing plate roll is provided with a non-image portion (a water retentive portion) bearing water and receiving no oil and an image portion (an inking portion) bearing the ink and repelling water.
Offset printing inks may usually be prepared by adding pigments to resinous varnishes comprising resins, solvents, drying oils such as linseed oil, and other additives.
The resinous varnishes for use in offset printing must have the following basic properties:
(1) In order to form images on a flat surface by keeping the ink in contact with water, they should maintain a surface balance between the water retentive portion and the inking portion;
(2) they should possess appropriate flowability as indicated by viscosity and yield value;
(3) they should exhibit good dispersibility with pigments;
(4) they should provide gloss and uniform impressions on the printed surface;
(5) they should quickly set and dry without causing blocking; and
(6) they should provide printed surfaces having good anti-friction property.
In order to satisfy the above properties required for the resinous varnishes for offset printing inks, the resins for use in the varnishes must meet the following requirements:
(1) they should have sufficient solubility in a high boiling point hydrocarbon solvent to be used for offset inks which have no aromatic content or a low aromatic content;
(2) they should have sufficient solubility in a drying oil such as linseed oil;
(3) they should have a high softening point, but not a high molecular weight; and
(4) they should have a polar group having pigment dispersibility (a polar group having good compatibility with the pigment).
As varnishes for printing ink, there have currently been used resinous varnishes prepared by dissolving a resin such as an alkylphenol resin, a rosin-modified phenol or a maleic acid resin in a drying oil such as linseed oil. Of these, the rosin-modified phenol resin has been most frequently used. A vehicle for printing ink, which comprises the rosin-modified phenol resin, a solvent and/or drying oil is excellent in quality; however, it has disadvantages in that the supply of rosin is not stable and its price is high because it is a naturally occurring substance. On the other hand, a so-called petroleum-derived resin which may be obtained by polymerizing cracked oil fractions in great quantities as by-products in petroleum or petrochemical industry, can offer advantages in that it can be constantly supplied and its price is stable; however, its quality is not satisfactory. Resins capable of being used as resins for printing ink have been little obtained from petroleum-derived resins.
It is disclosed in U.S. Pat. No. 3,084,147 that a resin soluble in a hydrocarbon solvent is obtained by thermally polymerizing dicyclopentadiene in the presence of an inert hydrocarbon solvent at a temperature of 250.degree.-350.degree. C. The dicyclopentadiene resin thus obtained cannot be utilized effectively for various uses because of its poor compatibility with and adherence to a variety of substances due to its lack of polar groups. An ink prepared from this resin by the addition thereto of various solvents and pigments is poor in dispersibility with the pigments and in its ability to stick to the printing material and provide gloss on printed surfaces, and it cannot provide a uniform printed surface. Accordingly, it cannot be used as a resin for offset printing inks.
Attempts have been made to provide a resin for use in the preparation of printing ink by adding an acrylic acid ester, maleic acid anhydride or the like to the above-mentioned dicyclopentadiene resin, hydrolyzing the mixture to give a carboxylic acid-containing resin, and then reacting the resulting resin with a polyhydric alcohol and a higher unsaturated aliphatic acid (Japanese Patent Public Disclosure No. 24,405/1972). An offset printing ink prepared from this resin may show decreased flowability and gloss on printed surfaces because of decreased solubility in petroleum-derived hydrocarbon solvents when the softening point of the base resin is elevated to bring its setting and drying times at printing to the same values conventional inks exhibit. This resin causes gelation and turbidity in the preparation of the varnish and gives an undesirable hue to the varnish. In order to improve its solubility in ink solvents, the resin must have a lower softening point. Accordingly, its setting and drying times become so long that it is no longer practical.
A resin obtained by thermally copolymerizing cyclopentadiene with maleic anhydride is disclosed in U.S. Pat. No. 2,608,550. This resin, when the amount of the maleic anhydride is lessened, usually exhibits an unfavorable elevation in its softening point and provides poor dispersibility with pigments and poor compatibility with other fillers; when maleic anhydride is added in larger amounts to improve these properties, the resin is apt to cause coloring and gelation, and its weather resistance is degraded.
It is further disclosed in U.S. Pat. No. 2,608,550 that a resin useful for ink is prepared by so-called alkyd modification comprising reacting a resin prepared by thermal polymerization of dicyclopentadiene with maleic anhydride, as a polybasic acid component, with a polyhydric alcohol and a drying oil. Since the alkyd resins thus prepared usually have high molecular weights, they have poor solubility in a high boiling point hydrocarbon solvent and a drying oil for use in ink. Thus, when formulated into an ink, the ink exhibits poor flowability and inferior gloss on the printed surface. Furthermore, the ink tends to disperse in a mist (so-called "misting"), thereby staining the printing paper so that it cannot be practically used.
In every case mentioned hereinabove, those resins are very poor in solubility in paraffinic solvents so that they are not applicable whatsoever to offset inks where a paraffinic solvent is used.
In order to provide offset ink compositions which are improved in the various properties referred to hereinabove, we have previously proposed offset ink compositions containing as vehicles resins obtainable by preparing a hydrocarbon resin by copolymerizing a particular five-membered ring compound having a conjugated double bond and/or a Diels-Alder addition product of the above five-membered ring compound with at least one component selected from the group consisting of turpentine oil, diisobutylene and nonene; reacting the hydrocarbon resin with an unsaturated carboxylic acid or an anhydride thereof to produce an acid-modified resin; and then thermally reacting the acid-modified resin with a phenol resin obtainable by condensation of a phenol having an alkyl group having 4 to 9 carbon atoms with formalin (Japanese Patent Application No. 131,665/1978).
We further proposed novel varnish compositions for use in offset inks, containing (a) a resin obtainable by reacting a hydrocarbon resin obtained by polymerizing a particular five-membered ring compound having a conjugated double bond and/or Diels-Alder addition product thereof with an unsaturated carboxylic acid and/or an anhydride thereof to provide an acid-modified resin, additionally reacting the resulting acid-modified resin with a higher saturated or unsaturated monohydric alcohol having 6 or more carbon atoms to yield an esterified resin, and then reacting the esterified resin with a phenol resin obtainable by reacting a phenol having an alkyl substituent having 4 to 9 carbon atoms with formalin; (b) a hydrocarbon solvent containing an aromatic content of less than 50% by weight and having a boiling point between 200.degree. C. and 350.degree. C.; and (c) drying oil (Japanese Patent Application No. 128/1979).